1. Field of the Invention
The present invention relates to line interface devices, and, in particular, to a storage charge reduction circuit that can be used to reduce the storage charge of a bipolar transistor of an input/output device.
2. Description of the Related Art
Data transceivers (TRANSmitter/reCEIVER) are typically used to interface Very Large Scale Integrated (VLSI) circuits to transmission mediums. The transmission mediums are generally collected together to form buses. The number, size and types of buses that are used in a digital system may be designed for general-purpose applications or according to a more specific, industry standard data-communications configuration. One such industry standard is the so-called IEEE 896.1 Futurebus+ standard. The Futurebus+ standard provides a protocol for implementing an internal computer bus architecture.
The transmission mediums are typically traces which are formed on the printed circuit board (PCB) substrates of daughter and mother boards. Microstrip traces and strip line traces can be employed to form transmission lines having characteristic impedances on the order of about 50.OMEGA.-70.OMEGA.. Such transmission lines usually have their opposite ends terminated in their characteristic impedance. Because of the parallel resistive terminations, the effective resistance of the transmission line may be as low as 25.OMEGA.-35.OMEGA..
A data transceiver is a read/write terminal capable of transmitting information to and receiving information from the transmission medium. A transceiver typically includes a line driver stage (or simply "driver") and a receiver stage (or simply "receiver"). The common purpose of transmission line drivers and receivers is to transmit data quickly and reliably through a variety of environments over electrically long distances. This task is complicated by the fact that externally introduced noise and ground shifts can severely degrade the data.
Drivers amplify digital signal outputs from the VLSI circuitry so that the signals can be properly transmitted on the transmission medium. Receivers are typically differential amplifiers that receive signals from the transmission medium and provide outputs to the VLSI circuitry that are representative of digital information received from the medium.
Conventional drivers usually include level shifting capability to provide compatibility with different integrated circuit technologies. Specifically, before a driver transmits a signal across a transmission medium, the driver changes the nominal voltage swing (or the "dynamic signal range") utilized by the VLSI circuitry, e.g., CMOS, TTL, ECL, etc., to a different voltage swing that is utilized by the transmission medium. Thus, a driver not only amplifies a digital signal, but it changes the nominal voltage swing of the signal as well.
A different nominal voltage swing is normally used when transmitting data across a transmission medium in order to conserve power. Specifically, the power internally dissipated by the driver is proportional to the nominal voltage swing of the binary signal it applies to the transmission line. Therefore, power dissipation is reduced if the driver transmits a signal having a relatively small voltage swing over the transmission line.
It has become common for signals to be transmitted over transmission lines at BTL (Backplane Transceiver Logic) signal levels. The signal level standard is denoted "Backplane" because BTL has been used primarily in the backplane buses of mother boards. Because the nominal voltage swing of BTL is 1.0 Volt (logic low) to 2.1 Volts (logic high), power dissipation is less than it would be if the signals were transmitted over the transmission lines at CMOS (0 Volts to 3.3 Volts, or, 0 Volts to 5 Volts) or TTL (0 volts to 3.5 Volts) signal levels.
FIG. 1 illustrates a prior art BTL driver 20. The driver 20 receives CMOS level signals at input V.sub.IN and outputs BTL level signals to a transmission line 22 at output V.sub.OUT. The driver 20 is implemented with bipolar transistors Q1, Q2, Q3, Q4, and Q5. Transistors Q1, Q2, and Q4 are Schottky bipolar transistors. Bipolar technology is attractive for implementing I/O devices, such as line or bus drivers, because of its unique high current gain characteristic. High current gain is important in a bus system, such as future bus backplane, because the driver 20 must be capable of driving the transmission line in both unloaded and loaded conditions.
Transistors Q2, Q3, Q4, and Q5 form an input stage 24 which controls the output transistor Q1. The input stage 24 charges and discharges the base of transistor Q1 in order to switch it on and off. FIG. 2 shows the input V.sub.IN and corresponding output V .sub.OUT waveforms for the driver 20. The driver 20 is an inverter. When the input V.sub.IN is low, the output transistor Q1 does not conduct current which causes the output V.sub.OUT to be high. When the input V.sub.IN is high, the output transistor Q1 conducts current which causes the output V.sub.OUT to go low.
The output V.sub.OUT falling edge propagation delay time T.sub.pHL is defined as the time between the 50% level of the input V.sub.IN rising edge and the 50% level of the output V.sub.OUT falling edge. The falling edge propagation delay time T.sub.pHL may also be referred to as the output V.sub.OUT turn-on time T.sub.ON because the output transistor Q1 is turning on during this time period. The output V.sub.OUT rising edge propagation delay time T.sub.pHL is defined as the time between the 50% level of the input V.sub.IN falling edge and the 50% level of the output V.sub.OUT rising edge. The rising edge propagation delay time T.sub.pHL may also be referred to as the output V.sub.OUT turn-off time T.sub.OFF because the output transistor Q1 is turning off during this time period. The delay times T.sub.pHL and T.sub.pHL should each normally be less than or equal to 5.0 nanoseconds (ns).
It is advantageous for the driver 20 to have a tight skew time T.sub.skew. The skew time T.sub.skew is given by the equation: EQU T.sub.skew =T.sub.pHL -T.sub.pLH ( 1)
The skew time T.sub.skew should typically be less than or equal to 2.0 ns over commercial voltage supply V.sub.CC and temperature ranges. Thus, the difference between the propagation delay times T.sub.pHL and T.sub.pLH should preferably be small and remain small during voltage supply V.sub.CC and temperature variations.
Because the output transistor Q1 of the driver 20 is a bipolar transistor, the propagation delay times T.sub.pHL and T.sub.pLH are affected by the bipolar transistor's current gain and storage time. Specifically, FIG. 2 illustrates the driver 20 output V.sub.OUTht during an increase in temperature. Temperature variations may be in the form of ambient temperature variations, i.e., variations in the temperature of the air surrounding integrated circuits, and/or junction temperature variations, i.e., variations in the temperature of the silicon in an integrated circuit. Ambient temperature variations can cause junction temperature variations, and vice versa.
The increased temperature causes the beta .beta..sub.Q1 of transistor Q1 to increase. An increase in .beta..sub.Q1 causes an increase in the excess base current I.sub.xbQ1 of transistor Q1 which significantly increases transistor Q1's base over-drive. Such an increase in transistor Q1's base over-drive causes transistor Q1 to switch on more quickly which decreases the falling edge propagation delay time T.sub.pHL.
However, the increase in the excess base current I.sub.xbQ1 of transistor Q1 due to the temperature increase causes more storage charge to accumulate between the collector and base (the collector-base region) of transistor Q1. The accumulation of storage charge in the collector-base region causes transistor Q1 to switch off more slowly which increases the rising edge propagation delay time T.sub.pLH. Thus, when temperature increases, the skew time T.sub.skew tends to increase because the falling edge propagation delay time T.sub.pHL decreases and the rising edge propagation delay time T.sub.pHL increases.
On the other hand, FIG. 2 illustrates the driver 20 output V.sub.OUTlt during a decrease in temperature. The decreased temperature causes .beta..sub.Q1 of transistor Q1 to decrease which decreases the excess base current I.sub.xbQ1 of transistor Q1. This decreases transistor Q1's base over-drive. Such a decrease in transistor Q1's base over-drive causes transistor Q1 to switch on more slowly which increases the falling edge propagation delay time T.sub.pHL. However, the decrease in the excess base current I.sub.xbQ1 of transistor Q1 causes less storage charge to accumulate in transistor Q1's collector-base region. The reduction in the accumulation of storage charge causes transistor Q1 to switch off more quickly which decreases the rising edge propagation delay time T.sub.pLH. Thus, when temperature decreases, the skew time T.sub.skew tends to increase because the falling edge propagation delay time T.sub.pHL increases and the rising edge propagation delay time T.sub.pHL decreases.
Variations in the voltage supply V.sub.CC have a similar effect on the driver 20's skew time T.sub.skew.
In an attempt to provide some control over the skew time T.sub.skew during temperature and voltage supply V.sub.CC variations, a Schottky diode D17 is connected between resistor R18 and ground. The Schottky diode D17 is intended to compensate for decreases in the base-emitter voltage V.sub.beQ1 in order to maintain a relatively constant voltage V.sub.R18 across resistor R18 during temperature increases. By maintaining a relatively constant voltage V.sub.R18, a relatively constant current I.sub.R18 is maintained through resistor R18 which is supposed to divert some of the excess base current I.sub.xbQ1 through resistor R18 to ground. The diversion of some of the excess base current I.sub.xbQ1 is supposed to prevent a large accumulation of storage charge in transistor Q1's collector-base region. By preventing a large accumulation of storage charge, the output transistor Q1 is able to switch off at its normal speed resulting in the rising edge propagation delay time T.sub.pLH remaining fairly constant. Without the Schottky diode D17, the voltage V.sub.R18, and thus the current I.sub.R18 conducted by resistor R18, would decrease during temperature increases which would mean that very little, if any, of the excess base current I.sub.xbQ1 would be diverted.
However, the Schottky diode D17 fails to provide control over the skew time T.sub.skew during temperature and voltage supply V.sub.CC variations. As mentioned above, when temperature increases, the excess base current I.sub.xbQ1 of transistor Q1 increases. Even with the Schottky diode D17, the current I.sub.R18 tends to decrease during temperature increases. The only effect of the Schottky diode D17 is to cause the current I.sub.R18 not to decrease quite as much as it would if the Schottky diode D17 were not present. Because during temperature increases the current I.sub.R18 decreases, or at best remains relatively constant, very little, if any, of the increased excess base current I.sub.xbQ1 is actually diverted to ground. The failure of the excess base current I.sub.xbQ1 to be diverted causes a large amount of storage charge to accumulate in transistor Q1's collector-base region.
Thus, there is a need for a circuit that can be used with a bipolar input/output device to maintain a relatively small skew time T.sub.skew during temperature and/or voltage supply V.sub.CC variations.